1. Field of the Invention
The present invention relates to a current cutoff mechanism of a cell, particularly relates to a current cutoff mechanism of a cell used for securing safety when pressure inside a cell case is increased
2. Description of the Related Art
Heretofore, a sealed lithium ion secondary cell inside the case of which an electrolyte including an organic solvent for example is injected and which can be charged are widely used for a power source for portable equipment such as a mobile telephone and a personal computer
However, if such a cell is excessively charged when it is charged again or is charged with larger current than defined current, abnormality occurs in the cell, gas is generated in a cell case, pressure and temperature in the cell case rise, the cell case is swollen, further a crack is formed, an inside electrolyte leaks outside and the leaked electrolyte sometimes has an adverse effect on equipment in which the cell is built.
If the use of the above cell is continued in the above abnormal state even if the above leaked electrolyte does not have an adverse effect on the equipment, the swell of the cell progresses, the cell case may burst and the use of the abnormal cell is required to be stopped promptly.
A conventional type current cutoff mechanism of a cell for preventing the burst of such a cell will be described as a circular cell shown in FIGS. 10 and 11 below. First, in a cell case 1, a cell lid 3 attached by riveting the periphery of the cell lid via a gasket 2 or welding in order to seal the inside of the cell case 1 and an actuator 4 which can be displaced above and below in the lower part of the cell lid 3 are respectively arranged.
An air vent 3a is formed in the cell lid 3 and when the actuator 4 in the lower part is displaced and cleaves its way upward air in a part between the cell lid 3 and the actuator 4 is emitted outside from the air vent 3a.
In the actuator 4, a circular relief valve 4a is formed by drawing and in the relief valve 4a, a projection 4b protruded downward in the drawings is formed in the center and a radial groove 4c is formed on the surface around the projection 4b.
An insulating plate 5 is arranged below the actuator 4 and in the insulating plate 5, a hole 5a for inserting the projection 4b of the relief valve 4a and an air vent 5b are formed.
A lead fixing member 6 is arranged below the above insulating plate 5 and in the lead fixing member 6, a hole 6a for communicating with the above hole 5a of the insulating plate 5 and an air vent 6b communicating with the air vent 5b of the insulating plate 5 are respectively provided.
The projection 4b of the relief valve 4a is inserted into the respectively holes 5a and 6a of the above insulating plate 5 and the lead fixing member 6 and a thin lead 7 made of a metallic plate is attached to the end of the projection 4b.
The thin lead 7 is provided with a junction 7a bonded to the end of the projection 4b by welding, and the actuator 4 and the lead 7 are electrically connected. The other end of the lead 7 is connected to a generating element 8 in the lower part in the drawings and a conductive path is formed between the generating element 8 and the cell lid 3.
If an abnormality occurs inside the cell and pressure inside the cell case 1 is increased, the pressure of gas is increased and the gas flows from the air vents 5b and 6b as shown by an arrow A in FIG. 11. The application force which pushes up the rear of the relief valve 4a is applied.
When concentrated stress is applied to the junction 7a of the lead 7 by application force applied to the relief valve 4a and the concentrated stress is larger than the shearing stress of the junction 7a, the junction 7a is disconnected from the lead 7, electrical connection between the lead 7 and the actuator 4 is cut off and the conductive path of the above cell is cut off.
The flow of current inside the cell is cut off by the above cutoff of the conductive path, the increase of pressure inside the cell case 1 is prevented and the cell can be prevented from bursting beforehand.
Portable equipment using such a conventional type cell (such as a mobile telephone) has become more and more miniaturized and the need has increased for the miniaturization and the thinning of the cell used for such portable equipment.
However, as the above conventional type current cutoff mechanism is formed by one plate though the lead 7 is thin, the application force of the actuator 4 varies widely depending upon variation in the thickness of the lead 7 and it is difficult to securely disconnect the junction 7a from the lead 7 when the internal pressure of the cell is increased up to a predetermined value.
As the junction 7a is disconnected as if it were torn off when the internal pressure of the cell is increased and the junction 7a is disconnected from the lead 7, a burr like a beard is made on the section of a disconnected hole 7b of the junction 7a after the junction is disconnected from the lead and it is difficult to precisely and completely cut off the conductive path between the actuator 4 displaced upward and the lead 7.